Heart failure (HF) is a major cause of morbidity and mortality, contributing signi?cantly to global health expenditure. Sudden death due to arrhythmia is responsible for over 50% of deaths among HF patients; however, the mechanisms linking HF-induced molecular remodeling to increased sudden death risk remain poorly understood. This has resulted in ineffective pharmacologic therapy for preventing sudden arrhythmic death and in inadequate approaches to arrhythmia risk strati?cation of HF patients. The overall objective of the proposed research is to explore a novel set of mechanisms by which HF remodeling, from the sub-cellular microdomain to the whole heart, leads to increased risk of lethal arrhythmias in human HF. Speci?cally, we propose to investigate how the impact of the degradation of myocyte microdomains on L-type Ca channel and cellular function is ampli?ed regionally by the heterogeneities in electrophysiological remodeling and adrenergic innervation as well as by the disease-induced remodeling in ventricular structure to produce i) arrhythmia triggers and ii) their degeneration into ventricular ?brillation (VF). The project presents an integrated experimental/computational approach to arrhythmogenesis in human HF. Super-resolution scanning patch clamp will provide novel insight into how disruption of sub-cellular compartments affects L-type Ca channel functioning in the HF cell. This data will be used as input into an integrative human HF myocyte model, which following validation, will be implemented in organ-level HF models. Protein and microstructure distribution data informing the organ-level models will be gathered in experiments with explanted HF human hearts. Model components will be combined with MRI scans of HF human heart geometry/structure to develop multiscale HF ventricular models which will then be used to determine the mechanisms responsible for the formation of 1) hot spots, from which triggered activity emanates, and 2) arrthythmogenic substrates at heart rates near rest, causing the degradation of triggered activity into VF. Simulation results regarding the arrhythmogenic substrate and VF likelihood at these heart rates will be validated in a clinical study of HF patients. Completion of the studies proposed here will result in a greater understanding of the mechanisms leading to arrhythmias and sudden death in human HF. Such mechanistic understanding is expected to reduce the impact of HF on its victims and on the health-care system 1) by suggesting targeted and effective new molecular therapies, and 2) by leading to new and improved approaches to arrhythmia risk strati?cation of HF patients.